Evolution, a process of change through time, is what links together the enormous diversity of the living world. A lot of evidence is present that indicates that the earth has had a very long history and that all living things arose in the course of that history from earlier, more simpler forms. In other words, all species have descended from other species and all living things share common ancestors in the past. Basically, organisms are what they are because of their history. Today there are many theories and possibilities related to evolution which contribute to our understanding of the process. Our planet was born 4.6 billion years ago as a great cloud of dust and gas condensed into a sphere. As gravity pulled this great cloud tightly together, heat from great pressure and radioactivity melted the planet's interior and most of its mass. For millions of years after this, strong volcanic activity all over the planet shook the earth's crust. At the same time, the earth was showered by a very strong meteor shower. From studying volcanoes, it is known that eruptions pour out carbon dioxide, nitrogen, and other gases. It is also known that meteorites carry water, in the form of ice, and many carbon containing compounds. That might suggest that the combination of volcanic activity and a constant shower of meteorites released the gases that created the Earth's atmosphere. Geologists believe that the earth's early atmosphere contained water vapor, carbon monoxide, carbon dioxide, hydrogen, and nitrogen. It also may have contained ammonia and methane. It did not contain oxygen, which is the main reason why the Earth could not have supported life. As for oceans, they couldn't have existed at first because the Earth's surface was extremely hot. But about 3.8 billion years ago, the Earth's surface cooled enough for water to remain a liquid on the ground. Thunderstorms wet the planet for many years and oceans began to fill.